DISCONTINUOUS IGNEOUS ADDITION AND EARLY MARGIN SEGMENTATION ALONG THE EASTERN NORTH AMERICAN MARGIN’S RIFT TO DRIFT TRANSITION

We used wide-angle marine active source seismic data from the 2014-2015 GeoPRISMS Eastern North American Margin (ENAM) Community Seismic Experiment (CSE) to investigate along-strike changes in igneous addition beneath the East Coast Magnetic Anomaly (ECMA). The ENAM formed during the breakup of Pangea ~200 myr ago and its crustal structure records extension and magmatism during rifting, as well as the rift to drift transition. Here, previous strike-perpendicular imaging showed igneous addition to the margin as seaward dipping reflectors and high velocity lower crust (HVLC) but along-strike variability implied by potential field anomalies could not be constrained without high resolution along-strike imaging. Constraints on along-strike crustal structure of the ENAM can provide insight into small scale processes that may be critical for a successful rift to drift transition and that influence later mid ocean ridge segmentation.

Tomographic modeling from ocean bottom seismometer data on lines 4A and 4B of the ENAM CSE shows a P-wave velocity structure resembling extremely thick oceanic-type crust. We observe minimal along-strike variations in crustal thickness, but substantial variations in the extent of HVLC. Within the HVLC sections, velocities at the base of the crust reach ~7.5 km/s, while two ~30 km-wide gaps are present where velocities only reach ~7.0 km/s. The gaps in HVLC align with isostatic gravity lows and with extrapolated Mid-Atlantic Ridge fracture zones. Mantle melting models indicate that the >7.5 km/s HVLC most likely represents heavy igneous intrusion into extended continental crust while the HVLC gaps most likely represent nearly unaltered extended continental crust. The along-strike intrusion variability also affects the volume of igneous addition, reducing it by up to 76% from previous whole margin estimates. Our along-strike observations closely match fracture zone formation modeling from the less magmatic Woodlark Basin where magmatic centers undergo the rift to drift transition and then connect into a stable ridge configuration. It is unclear what influenced segmentation of the Pangean rift, but it appears that the segmentation of the early rift resulted in a lower volume of igneous addition and directly influenced Mid-Atlantic Ridge segmentation.